This invention relates to a purifying process for technical grade cellulose ethers using aqueous salt solutions of polybasic inorganic acids and/or organic polycarboxylic acids.
Technical grade cold-water-soluble cellulose ethers, such as methyl cellulose, ethyl cellulose and mixed ethers of cellulose, such as methylhydroxypropyl cellulose, methylhydroxyethyl cellulose, methylhydroxybutyl cellulose, etc., contain considerable amounts of alkali metal halides, especially sodium chloride, due to their preparation by reaction of alkali metal cellulose with alkylating agents containing halides, such as methyl chloride, ethyl chloride, etc. The amount of alkali metal halides can amount to up to 50% by weight of the crude product, and is usually at least 25% by weight of the crude product. The content of alkali metal halides in the cellulose ether interferes with most intended purposes, and the crude cellulose ethers must be freed from alkali metal halides. The fact that the cellulose ethers are soluble in cold water but not in hot water is utilized in the conventional separation of the alkali metal halide impurities. The alkali metal salts, especially sodium chloride, are washed out with hot water. After separating the wash liquid, the products purified in this manner are dried and ground into powder or changed into a granulate.
This known purification process for cellulose ethers requires large amounts of energy and is expensive.